Re: [PATCH] atomic: add atomic_inc_not_zero_hint()

Andrew Morton
2010-11-05T17:21:16Z

On Fri, 05 Nov 2010 17:53:00 +0100
Eric Dumazet <eric.dumazet@gmail.com> wrote:

> Andrew
>
> atomic_inc_not_zero() / atomic_add_unless() ... are spreaded in many
> arch/xxx/include/asm/atomic.h, but they are generic.
>
> Apparently there is no centralization of some common atomic features...
>
> What do you think adding a new file so that we can move common
> definitions in it ?
>
> Thanks
>
> [PATCH] atomic: add atomic_inc_not_zero_hint()
>
> Followup of perf tools session in Netfilter WorkShop 2010
>
> In network stack we make high usage of atomic_inc_not_zero() in contexts
> we know the probable value of atomic before increment (2 for udp sockets
> for example)
>
> Using a special version of atomic_inc_not_zero() giving this hint can
> help processor to use less bus transactions.
>
> On x86 (MESI protocol) for example, this avoids entering Shared state,
> because "lock cmpxchg" issues an RFO (Read For Ownership)

It totally makes sense to add include/linu/atomic.h for common things.
Perhaps there's already code in arch/*/include/asm/atomic.h which
should be hoisted up there. But that can't reliably be done until a
million files have had their #includes switched :(

> Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
> Cc: Christoph Lameter <cl@linux-foundation.org>
> Cc: Ingo Molnar <mingo@elte.hu>
> Cc: Andi Kleen <andi@firstfloor.org>
> Cc: Arnaldo Carvalho de Melo <acme@infradead.org>
> Cc: David Miller <davem@davemloft.net>
> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> Cc: Nick Piggin <npiggin@kernel.dk>
> ---
> include/linux/atomic.h | 29 +++++++++++++++++++++++++++++
> 1 file changed, 29 insertions(+)
>
> diff --git a/include/linux/atomic.h b/include/linux/atomic.h
> new file mode 100644
> index 0000000..c04327a
> --- /dev/null
> +++ b/include/linux/atomic.h
> @@ -0,0 +1,29 @@
> +#ifndef _LINUX_ATOMIC_H
> +#define _LINUX_ATOMIC_H
> +#include <asm/atomic.h>
> +
> +/**
> + * atomic_inc_not_zero_hint - increment if not null
> + * @v: pointer of type atomic_t
> + * @hint: probable value of the atomic before the increment
> + *
> + * This version of atomic_inc_not_zero() gives a hint of probable
> + * value of the atomic. This helps processor to not read the memory
> + * before doing the atomic read/modify/write cycle, lowering
> + * number of bus transactions on some arches.
> + * Note: hint MUST not be 0 !
> + */

It's quite unobvious *why* `hint' cannot be zero. Can you please add
the reasoning to the comment? Also, the local `hint' should be
referred to as "@hint" in a kerneldoc comment.

> +static inline int atomic_inc_not_zero_hint(atomic_t *v, int hint)
> +{
> + int val, c = hint;
> +
> + do {
> + val = atomic_cmpxchg(v, c, c + 1);
> + if (val == c)
> + return 1;
> + c = val;
> + } while (c);
> +
> + return 0;
> +}

Should/could this have implemented a more general
atomic_add_not_zero_hint() and made atomic_inc_not_zero_hint() a
wrapper around that?

Also, it might make sense to add "#ifndef atomic_inc_not_zero_hint"
wrappers around this function so that the arch can implement an
overriding custom version. And that becomes a general rule as more
functions are added to include/linux/atomic.h.